1. Field of the Invention
The present invention relates to a technique for creating a mini time key from a time key with which decryption at a specific time is enabled, and in particular to a technique for reducing the size of the time unit allocated for decryption using a mini time key without a corresponding increase in the number of time keys.
2. Description of the Related Art
In the present invention, a mini time key creation method and a management (transmission and maintenance) method for its use will be explained. Generally, the number of time keys to be managed is increased in order to reduce the unit time allocated for decryption. However, according to the present invention, the unit time can be shortened without a corresponding increase in the number of time keys that are to be managed.
The time key is used for a system to prevent the decryption of data until a specific time has been reached. In this system, a time key manager that manages a time encryption key keeps a time decryption key secret until a specific time, and after that, releases the time decryption key for public use.
In order to shorten the unit time allocated for decryption, many time keys that correspond to the unit time must be created. For a unit time of one day, for example, 365 time keys must be created and managed for one year (a time key for Jan. 16, 1998 or a time key for Jan. 17, 1998). However, for a unit time of one minute, 525,600 time keys must be created and managed for one year (a time key for 10:28 AM on Jan. 16, 1998 or a time key for 10:29 AM on Jan. 16, 1998). As is described, the number of time keys to be managed is normally increased in order to reduce the unit time allocated for decryption. Since a system having the highest security is required for the management of the time keys, the creation of as small as possible number of time keys is desired. Taking into consideration an application that uses a time key, the shorter the unit time is allocated for decryption, the more flexibly can the operation be performed.
A time key employing asymmetric key encryption is described in “Secure Electronic Sealed-Bid Auction Protocol With Public Key Cryptography,” M. Kudo, IEICE Trans. Fundamentals Of Electronics, Communications And Computer Sciences, Vol. E81-A, No. 1, 1998. And a time key employing symmetric key encryption is described in “Time-Lock Puzzles And Timed-Release Crypto,” R. L. Rivest, A. Shamir and D. A. Wagner, MIT Laboratory For Computer Science, pp. 1-9, 1996 Time-Lock Puzzles. Both references are concerned with the unit time key, and do not teach the method of the present invention whereby a mini time key is created from a unit time key in order to reduce the time unit allocated for decryption without a corresponding increase in the number of time keys.
It is, therefore, one object of the present invention to provide a method and a system for creating a mini time key from a time key.
It is another object of the present invention to provide a method and a system for performing encryption using a mini time key and a unit time key.
It is an additional object of the present invention to provide a method and a system for transmitting a mini time key and a unit time key.
It is a further object of the present invention to provide a method and a system for decrypting encrypted data using a mini time key and a unit time key.
It is still another object of the present invention to provide a time key server with which a user can freely use a mini time key and a unit time key.
It is a still additional object of the present invention to provide a time key management method whereby a time key management function does not have to manage an enormous number of time keys when using a mini time key and a unit time key, and a time key management system.